The present invention relates to a method and apparatus for irradiating products to achieve a radiation dose distribution that satisfies specified dose uniformity criteria throughout the product.
The treatment of products using radiation is well established as an effective method of treating materials such as medical devices or foodstuffs. Radiation processing of products typically involves loading products into totes and introducing a plurality of totes either on a continuous conveyer, or in bulk, into a radiation chamber. Within the chamber the product stacks pass by a radiation source until the desired radiation dosage is received by the product and the totes are removed from the chamber. As a plurality of products, typically within totes, are present in the chamber at a given time, the radiation processing parameters affect all of the product within the chamber at the same time.
One common problem in the radiation processing of products is that the effectiveness of radiation processing is sensitive to variations in product density and geometry, and product source geometry. If a radiation chamber is loaded with totes comprising products with a range of densities and geometries, certain products will tend to be over-exposed to the radiation, while others do not achieved the required dose, especially within the central regions of the product. To overcome this problem the radiation chamber is typically loaded with products according to a specified and validated configuration so that the processing of the products satisfies a specified dose uniformity criteria. However, this is not always possible as some product package configurations are not compatible with achieving a good dose uniformity when irradiation is carried out in the conventional manner.
Products of a large dimension, and high density suffer from a high dose uniformity ratio (DUR) across the product. A relatively even radiation dose distribution (small DUR) is desirable for all products, but especially so for the treatment of foods, such as red meats and poultry. In treatment of these products, an application of an effective radiation dose to reduce pathogens at the centre of the stack is often limited by associated undesirable sensory or other changes in the periphery of the product stack as a result of the higher radiation dose delivered to material in this region of the product. A similar situation may arise during the radiation serialization of medical disposable products, a majority of which may be made from plastic materials. In these cases, the maximum permissible radiation dose in a product may be limited by undesirable changes in the characteristics of the plastics, such as increased embrittlement of polypropylene or decoloration and smell development of polyvinyl chloride. In order to adequately and thoroughly treat product stacks of such products with radiation processing, a relatively even radiation dose distribution characterized by a low DUR must be delivered throughout the product stack.
Radiation processing of materials and products has most often been accomplished using electron beams, gamma radiation or X-rays. A major drawback to electron beam processing, is that the electron beam is only capable of penetrating relatively shallow depths (i.e. cm) into product, especially high density products such as food stuffs. This limitation reduces the effectiveness of electron beam processing of bulk or palletized materials of high density. Gamma radiation is more effective in penetrating products, especially those of a higher density or larger dimensions, compared with electron beam. Most gamma sources are based on radioactive nuclides such as colbat-60. Kock and Eisenhower (National Research Council of the National Academy of Sciences Publication #1273; 1965) discuss the merits of different types of radiation processing for the purposes of food treatment. The article suggest that photons are the preferred source for treating large product stacks because of the greater ability of photons to penetrate the product.
U.S. Pat. No. 4,845,732 discloses an apparatus and process for producing bremsstrahlung (X-rays) for a variety of industrial applications including irradiation of food or industrial products. An alternate device for the production of X-rays is disclosed in U.S. Pat. No. 5,461,656 which also discloses X-ray irradiation of a range of materials. U.S. Pat. No. 5,838,760 and U.S. Pat. No. 4,484,341 teach a method and apparatus for selectively irradiating materials such as foodstuffs with electrons or X-rays. None of these documents discloses an apparatus or methods to deliver a relatively even radiation dose distribution, especially in large product stacks of high density, so that a low DUR is achieved in treated products.
U.S. Pat. No. 4,561,358 discloses an apparatus for conveying articles within a tote (carrier) through an electron beam. The invention teaches of a carrier that is capable of reorienting its position as the carrier approaches the electron beam. An analogous system is disclosed in U.S. Pat. No. 5,396,074 wherein articles are transported past an electron beam on a process conveyor system. The conveyor system provides for re-orientation of the carrier so that a second side (opposite the first side) of the carrier is exposed to the radiation source. The carrier is further defined in U.S. Pat. No. 5,590, 706. A similar electron beam irradiation device is disclosed in U.S. Pat. No. 5,994,706. An apparatus to optimize the dosage of electron beam radiation within a product are given in U.S. Pat. No. 4,983,849. The apparatus includes placing cylindrical or plate dose attenuators between the radiation beam and product. The attenuators comprise a moving, perforated metal plate (or cylinder) scatter the radiation beam and reflect non-intersecting electrons thereby increasing dosage uniformity.
U.S. Pat. No. 5,554,856 discloses a radiation sterilizing conveyor for sterilizing biological products, food stuffs, or decontamination of clinical waste and microbiological products. Products are placed on a disk-shaped transporter and rotated so that the products are exposed to a field of accelerated electrons. A similar apparatus for electron beam serialization of biological products, foodstuffs, clinical waste and microbiological products is also disclosed in U.S. Pat. No. 5,557,109. Products are placed in a recess or pocket of a manipulator which is slid horizontally into a cavity until the products are aligned with a path of an electron beam housed within the serialization unit.
In the prior art systems described above, there are limitations in the ability to deliver a relatively flat dose distribution (low DUR) throughout a product or product stack since no method is provided to compensate for the different dose received by the exterior and interior portions of the product stack. This therefore results in the outer portions of a product to receive a much higher radiation dose than that received within the product stack.
U.S. Pat. No. 4,029,967 and U.S. Pat. No. 4,066,907 disclose an irradiation device for the uniform irradiation of goods by means of electro-magnetic radiation having a quantum energy larger than 5 KeV. Products to be irradiated (including medical articles, feedstuffs, and food) rotate on turntables and are partially shielded from a radiation source by shielding elements. There is no discussion of optimizing the geometry of the radiation beam relative to the product stack, or modifying the spacing of the shield elements in order to optimize the DUR within a product. As a result, products with different densities are still subject to a wide range of DUR as is the case with other prior art systems. U.S. Pat. No. 5,001,352, also discloses a similar apparatus comprising product stacks that rotate on turntables, positioned around a centrally disposed radiation source, and shielding elements that reduce lateral radiation emitting from the source. A shielding element comprising a plurality of pipes that are fluid filled thereby permitting flexibility in the form of the shielding element is also discussed. However, there is no guidance as to how this or the other shielding elements are to be positioned in order to attenuate the radiation beam relative to the product stack in order to optimize the DUR within the product. Nor is there any discussion of any real-time adjustment of shielding elements to optimize the dose distribution received by a product that accounts for alterations in product densities.
A major limitation with the prior art irradiation systems is that it is difficult to obtain a relatively even radiation dose distribution (low DUR) throughout a product or product stack. For example, in systems which irradiate products from only one side, the material irradiated at the periphery of the product and closest to the irradiation source receives a high radiation dose relative to the product located at the center regions of the product stack, and further away from the radiation source resulting in a high DUR. Even with systems that irradiate products from multiple sides, the material irradiated at the periphery of the product typically receives a higher dose of radiation than the material located at the centre of the product since the radiation method is not optimized for the product stacks. Consequently, the product receives an uneven dose of radiation, characterized by a high DUR. Thus, prior art systems are limited in their ability to deliver a relatively flat dose distribution (low DUR) throughout a product or product stack. These limitations are more pronounced in larger products, with higher densities.
It is an object of the current invention to overcome drawbacks in the prior art.
The above object is met by the combinations of features of the main claims, the sub-claims disclose further advantageous embodiments of the invention.
The present invention relates to a method and apparatus for irradiating products to achieve a radiation dose distribution that satisfies specified dose uniformity criteria throughout the product.
According to the present invention there is provided a product irradiator comprising: a radiation source, an adjustable collimator, a turntable; and a control system. The radiation source may be selected from the group consisting of gamma, X-ray and electron beam radiation. Preferably, the radiation source is an X-ray radiation source comprising an electron accelerator for producing high energy electrons, a scanning horn for directing the high energy electrons and a converter for converting the high energy electrons into X-rays.
The present invention is also directed to the product irradiator as defined above which further comprises a detection system. The detection system measures at least one of the following parameters: transmitted radiation, instantaneous angular rotation velocity of the turntable, angular orientation of the turntable, power of the radiation beam, energy of the radiation beam, collimator aperture, width of the radiation beam, position of an auxiliary shield, offset of the radiation beam axis from axis of rotation of the product on the turntable, distance of the turntable from collimator, and distance of collimator from the source. Preferably, the detection system is operatively linked with said control system.
The present invention also pertains to a method of radiation processing a product comprising:
i) determine length, width, height and density of a product stack comprising the product;
ii) determining the width of a collimated radiation beam required to produce a low Dose Uniformity Ratio within the product;
iii) adjusting a collimator aperture to obtain the width determined in step ii); and
iv) rotating the product stack within the collimated radiation beam for a period of time sufficient to achieve a minimum required radiation dose within the product.
This method also pertains to the step of adjusting (step iii), wherein an angular velocity of the turntable may be adjusted. Furthermore, within the step of adjusting, the collimated radiation beam is collimated X-ray beam produced from high energy electrons generated by an electron accelerator, and power of the high energy electrons may be adjusted.
This invention also pertains to the method as defined above wherein during or following the step of rotating, is a step (step vi) of detecting X-rays transmitted through the product. Furthermore, during or following the step of detecting (step vi), is a step (step vii) of processing information obtained in the detecting step by a control system and altering, if required, of any of the following parameters: collimator aperture, distance between the turntable and collimator, turntable offset, position of auxiliary shield, angular velocity of the turntable, power of the high energy electrons.
The present invention also pertains to the use of an apparatus comprising a radiation source for producing radiation energy selected from the group consisting of x-ray, e-beam, and radioisotope, an adjustable collimator capable of attenuating first portion of the radiation while permitting passage of a second portion of the radiation, the second portion of radiation shaped by the adjustable collimator into a radiation beam, the radiation beam traversing a turntable capable of receiving a product stack, and a control system capable of modulating the adjustable collimator or any one or all irradiation system parameters as the product stack rotates on the turn-table, for delivery of a radiation dose producing a low dose uniformity ratio (DUR) within the product stack.
The present invention further pertains to a method of irradiating a product stack with a low dose uniformity ratio comprising, rotating a product stack in an X-ray radiation beam of width less than or equal to the diameter of the product stack and modulating the width of the radiation beam relative to the rotating product stack. Modulation of the width of the radiation beam may be effected by adjusting the adjustable collimator, the distance between the product stack and collimator, or the distance between the source and collimator, position of an auxiliary shield, or a combination thereof, as the product stack rotates in the radiation beam.
The present invention is directed to a product irradiator comprising:
i) an X-ray radiation source essentially consisting of an electron accelerator for producing high energy electrons, a scanning horn for directing the high energy electrons towards a converter, the converter for converting said high energy electrons into X-rays to produce an X-ray beam, the X-ray beam directed towards a product requiring irradiation;
ii) an adjustable collimator for shaping the X-ray beam;
iii) a turntable upon which the product is placed; and
iv) a control system in operative communication with the electron accelerator, the adjustable collimator and the turntable.
This invention also pertains to the product irradiator just defined further comprising a detection system in operative association with the control system. Furthermore, the turntable of the product irradiator may be movable towards or away from the adjustable collimator, or the turntable may be movable laterally, so that an axis of rotation of the product on the turntable is offset from the X-ray beam axis. The product irradiator may also comprising an auxiliary shield.
The present invention also pertains to the product as defined above, wherein the detection system measures at least one of the following parameters: transmitted X-ray radiation, instantaneous angular velocity of the turntable, angular orientation of the turntable, power of the high energy electrons, width of high energy electron beam, energy of the X-ray beam, aperture of the adjustable collimator, position of the auxiliary shield, offset of the radiation beam axis from axis of rotation of the turntable, distance of the turntable from collimator, and distance of the collimator from the radiation source.
This summary of the invention does not necessarily describe all necessary features of the invention but that the invention may also reside in a sub-combination of the described features.